In the application of such drill booms the hydraulic pilot cylinder means operates as a pump which consumes a portion of the energy of the hydraulic elevating cylinder means for providing the parallel displacement. Thus the hydraulic elevating cylinder means must be made more heavy than for a pure elevating function. A drill boom of the abovementioned type is disclosed in Canadian Pat. No. 886,975. Due to the heavy elevating cylinder means the attachment of the boom becomes relatively voluminous and its design, because of increasing load, becomes heavier.
It is an object of the invention to make use of the hydraulic pilot cylinder means as a component for obtaining elevating and lowering of the drill boom under the parallel displacement principle in question with the result that the hydraulic elevating arrangement can be made substantially more compact and can be associated with a lighter boom construction.